Vacuum Ultraviolet Photochemistry. II. Photolysis of Ethylene

Abstract

The direct photolyses of CH₂CD₂ and trans‐CHDCHD were carried out at room temperature in the vacuum ultraviolet region. Kr, Xe, and Hg resonance radiations were used for photolyses. Products at the Kr lines were mainly hydrogen, acetylene, ethane, and n‐butane. The ratio of acetylene/hydrogen was 2.8±0.2 at 1236 A. The photolysis of CH₂CD₂ gives 40% H₂, 40% HD, and 20% D₂. The acetylene was 12% C₂H₂, 62% C₂HD and 26% C₂D₂. The relative amounts of H₂, HD and D₂ depend upon whether a hydrogen molecule is eliminated from two (type I process) or one (type II process) carbon atom. The remaining vinylidene rapidly rearranges to acetylene. The photolysis of trans‐CHDCHD gives 17% H₂, 73% HD, and 10% D₂. This shows that hydrogen is formed from the trans as well as from the cis position and that the isotope effect for the type I process is slightly smaller than that for the type II process. Nearly free rotation of the molecule in its excited state is suggested. The percentages of the hydrogen isotopes produced are almost independent of exciting wavelength in both isotopic ethylenes. A comparison is made with Hg(³P₁)‐sensitized photolysis.